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Sick Eucalyptus Tree

I have 3 Coolibah eucalyptus trees in my backyard, all approximately 25 ft tall and 20 or more years old. One of the trees has suddenly changed dramatically. There are barely any green leaves, and it looks as if it has died. The other two trees are in close proximity (about 10 feet apart) yet are green and look fine. I don't know how to save it or why only one tree is 'sick'. I have inspected it for signs of bugs and have found no evidence of such. My only guess is that it has gotten a lot more water than the other two due to surrounding landscape changes.